In a continuous casting installation, the molten metal received at a ladle is poured into a mold through a tundish. At such a time, a certain amount of molten metal is kept stored within the tundish before starting the pouring of the molten metal into the mold at the beginning of casting, and after the flowing-up of impurities a nozzle is opened to start a pouring. This procedure is called a closed start, and these days this technique has begun to be introduced.
In the closed starting of a continuous casting tundish various systems have been proposed for injecting an inert gas to prevent the molten metal from solidification within the nozzle while storing the molten metal in the tundish by closing the nozzle hole of a sliding porous refractory, and a system (shown in FIG. 8--Patent Kokai No. 177952/85) in which a gas injecting ring is arranged, said ring having a plurality of small radial orifices provided toward the slide plate side from a groove formed along the outer periphery within the opening of a bottom plate brick to the center of the opening.
Said known systems have the following drawbacks:
a) The gas guiding pipe (made of copper or steel) is connected and sealed with the gas feeding holes (made of brick bored with fine orifices) by means of sealing material, mortar or soldering, and in the high temperature condition the gas sometimes leaks (shown with x marks in FIG. 8) from the joints so as not to be injected into the nozzle opening.
Further, practically the gas amount injected into the nozzle opening becomes irregular so that stable casting is often not operated whereby the products and quality are not uniform.
b) The making process requires additional working such as a cutting processing of brick, a boring processing of brick, and a jointing working of the gas guiding pipe and the gas injecting brick thereby to demand high cost.